A conventional jet pump assembly includes a body with three distinct regions, mainly an inlet or suction section, a throat or a mixing chamber of substantially uniform cross-sectional area throughout its length, and a diffuser which increases in cross-sectional area in the direction of flow. A nozzle is positioned in the inlet section to convert a high-pressure stream of driving fluid into a high-velocity, low-pressure jet of driving fluid which flows coaxially through the inlet section and into the mixing chamber. The high-velocity jet is at a much lower pressure than fluid surrounding the nozzle in the vicinity of the inlet section so driven fluid is sucked into the pump inlet by the jet. A converging housing on the inlet section and surrounding the nozzle directs the driven fluid or suction flow into the mixing chamber. Within the mixing chamber, the high-velocity jet of driving fluid gradually widens as an entrainment-mixing process takes place with the driven fluid or suction stream. The mixing transfers momentum from the jet driving stream to the driven suction stream, so static pressure in the combined stream rises. In theory, the mixing chamber ends after a uniform velocity profile is achieved, and this usually occurs shortly after the widening driving jet stream touches the mixing chamber walls. From the relatively small cross-sectional area mixing chamber, the merged driving and driven fluids flow into the diffuser of increasing cross section in the direction of flow, further increasing pump discharge pressure as the velocity of the merged fluids is reduced to convert the maximum amount of velocity head to static head or pressure.
Since the jet pump has no mechanical moving parts, it is well suited for forced circulation of coolant typically water, in a boiling water reactor vessel where long and trouble-free operation is required because of the radiation, which makes pump repair or replacement difficult and expensive.
Most boiling water reactors typically have a plurality of jet pumps mated in pairs. Each mated pair works in conjunction with its respective mated jet pump for recirculating the reactor coolant. A riser is connected to each mated pair for providing a passageway of the coolant to each jet pump of the mated pair. The riser includes a vertically oriented stem portion for receiving the coolant, and the stem portion branches into distinct U-shaped flow channels which respectively attach to a nozzle of the mated pair of pumps, as previously discussed.
Unfortunately, the jet pumps create a large and upwardly hydraulic force caused by the coolant thrusting up through the riser and against the interior of the two U-shaped flow channels. This hydraulic force is typically mitigated by a holddown assembly attached to the riser. The holddown assembly includes a pair of spaced apart arms which are respectively attached to opposite sides of the riser, and extend upwardly from the channels. A beam is placed between the arms which, in conjunction with the beam, form a generally U-shaped structure surrounding the top of the channels. A bolt extends through and threads into the beam and abuts the junction of the two channels for resisting the upward force of the coolant flowing through the channels. A rectangular shaped weld plate having an annular transverse cross section portion extending from its center portion slips over the bolt for preventing rotational movement of the bolt once it is threaded into the beam. The annular shaped transverse cross section portion receives the bolt, and the weld plate is welded onto the beam.
Although the presently known and utilized holddown assembly and weld plate are satisfactory, they are not without drawbacks. The holddown assembly is susceptible to cracking which is caused by the harsh environment of the boiling water reactor vessel. In addition, if the holddown assembly needs to be removed for maintenance and the like, the weldment of the weld plate must be grounded out by a suitable tool which renders it unusable after this removal. Therefore, a new weld plate must be installed when the holddown assembly is re-installed.
Consequently, a need exists for an improved jet pump assembly which eliminates damaging the weld plate during removal and eliminates stress corrosion cracking.